1. Field of the Invention
The present invention pertains to a system and method for measuring the forces, with high accuracy, between a railway wheel set and the railhead of underlying track such as the angle of attack when the track undergoes a shallow curve.
2. Statement of the Problem
The interaction between a set of railway wheels and the underlying track has been extensively studied. The angle of attack (AOA) is generally defined as the yaw angle between the wheels and the rails. AOA is a critical factor for assessing rail vehicle performance. For example, during curve negotiation, a larger value of AOA indicates a potential for the wheel set to climb the rails or to generate large gage spreading forces. In FIG. 1, a set 100 of wheels 40 and 50 are connected to axle 30 and moves M in the direction shown on outside rail 10 and inside rail 20. The leading wheel 50 is on outside rail 10 and the trailing wheel 40 is on inside rail 20. One measurement of angle of attack is the angle (AOA1) between the plane 60 of wheel 50 and the tangent 70 to the outside rail 10 upon which the leading wheel 50 is engaged. Angle of attack is also shown by the angle (AOA2) between line 80 which is normal to the tangent 70 and the axle centerline 90.
When AOA is zero, the rotational velocity 110 of the wheel set has equal magnitude and direction as the translational velocity 120 of the railway vehicle to which the wheel set is attached. This results in pure rotation of the wheels which converts to pure forward velocity of the railcar attached to the wheels. At the other extreme where AOA is large, the translational velocity 120 of a railroad vehicle is due to the rotational velocity 110 plus a lateral velocity 130 as shown in FIG. 1. In this scenario, the lateral forces FL which are a function of the lateral velocity 130 on wheel 50 as shown in FIG. 2 are great which may result in damage, higher maintenance, or possible derailment. FIG. 2 also shows the vertical force, FV of the wheel 50, on the outside rail 10.
In FIG. 3, the conventional relationship between AOA, FL and FV is generally illustrated as curve 300. Curve 300 is well known such as found in the following reference: Kalker, “Review of Wheel-Rail Rolling Contact Theories,” pages 77-92 of The General Problem of Rolling Contact AMD-40 Published by The American Society of Mechanical Engineers. AOA appears on the horizontal scale and the ratio of the FL to FV is shown on the vertical scale. When FL is zero and AOA is zero, the rotational velocity of the wheel set is converted directly to the forward velocity of the rail vehicle. This is shown as 310 in FIG. 3. In region 330, lateral creepage occurs, and the lateral forces, FL, increase as the value of AOA increases. Lateral creepage can be defined as translational velocity 110 minus lateral velocity 120 as a percent of translational velocity 100. In region 320, the amount of friction between the wheel and the surface of the rail causes gross slippage to occur. Normally the ratio of FL to FV saturates at u, the coefficient of friction 350 for curve 300. Curve 340, for example, can be a lubricated set of rails that has a lower coefficient of friction.
In FIG. 1, the track 10, 20 has a curvature and the AOA increases proportionally with the curvature. One rule of thumb for North American three-piece trucks approximates the degree of curvature for the track to the AOA in milliradians. For example, on a six degree curve, the leading axle has an AOA of six milliradians. For shallow curves (i.e., two degrees or less such as a radius greater than 1000 meters), the lateral forces are smaller since the AOA is small. One difficulty in measuring AOA in shallow curves is the presence of cross-talk. Cross-talk is caused by the vertical load on the railhead and by the shape of the railhead. Curves of four degrees or greater, result in more accurate lateral force measurements as cross-talk is minimal (as found with AAR130 rail and normal lateral prone three-piece trucks).
Systems are available which measure AOA. U.S. Pat. No. 5,368,260 uses a wayside range finder that incorporates a beam of laser light directed to the wheel so as to measure AOA1 between the plane 60 of the wheel and the tangent 70 of the track 10 as shown in FIG. 1. In order to do this, wheel detectors are placed on the track so that passage of a wheel can be detected which start and stop the range finder. In addition, an average velocity measurement occurs. The range finder generates a complete profile image as each wheel passes the wayside range finder. From this image, AOA is calculated. One such system, Wayside Inspection Devices, Inc., 4390 De Maisonneuve, Westmount, Quebec H3Z 1L5 Canada, uses lasers precisely positioned on the wayside of a track to carefully determine AOA based on reflected laser light. These systems claim to accurately provide angle of attack measurements within one milliradian (i.e., 3.44 arc minutes). Such systems, however, are expensive, require continued maintenance and supervision, and are prone to vandalism.
Another prior art approach uses a pair of vertical strain gages to measure the passage of a set of wheels over the rails at the position of the strain gage. Offer and Martin, Rugged Transducers for Measurement of Angle of Attack and Lateral Railhead Displacement, Technology Digest, August, 1992 (TD 92-010). The use of strain gages in an AOA measurement system results in a much less expensive system, one that is easy to maintain, and one that is not easily vandalized in comparison to laser systems. Such strain gage systems, however, do not have the accuracy in measuring AOA as laser systems and usually results in an accuracy of 3-4 milliradians.
In addition to the systems discussed above, AOA has also been measured with a vehicle-mounted system for a particular wheel set as the rail vehicle travels on the track. Mace et al., New Vehicle-Mounted Angle of Attack Measurement System, Technology Digest, February 1995 (TD 95-004). These systems are mounted to each wheel set and, therefore, are not suitable for wayside use for determining AOA for all wheel sets in a train.
The known optical, laser, and strain gage wayside systems and methods for measuring angle of attack result in a static AOA measurement which does not take into account the dynamic misalignment of the rails as the wheel sets pass over or when misalignment of the wayside measuring system occurs due to soil, rail, or tie shifting due to moisture, temperature, lateral train forces, etc.
A need exists for a system and method for measuring AOA which is inexpensive, rugged, less prone to vandalism, easier to maintain, and yet provides an AOA measurement over a range of ±50 milliradians with an accuracy of 1 to 3 milliradians. Furthermore, a need exists for such a system and method to dynamically measure AOA so as to compensate for any misalignment. Finally, a need exists to improve upon the earlier conventional approach using strain gages by better predicting when the wheel set crosses directly over the AOA strain gages.
A further need exists to remove cross-talk in shallow curves for AOA measurement systems to improve the accuracy of measurements. While the above is directed towards AOA measurement systems, it is to be understood that a need exists to remove cross-talk from any system and method measuring the forces between a railway wheel set and the railhead of underlying track.